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Notify a separate JVM to preform a task

So I am making a program that will only run one instance at a time and am doing so by using this solution
However now I would like to make it so that if the user trys to launch another instance it will consume that attempt and notify the current instance to show its gui.
Currently I am thinking about doing this by the use of a file. Upon the launching of a second instance, a file called show.stage will be created. When the other instance detects that file it will show its gui and delete the file.
I know this works but I was wondering if there was a more graceful way to do this.
Could I some how set a environment flag that the other instance could check for or maybe notify it via a socket listener, although those seem to be discouraged by others. I get the feeling creating the file will be the easiest and most robust way but I am open to any suggestions
This program will be running on normal windows.

If you don't want to use a lock file (which I think is a perfectly good solution), you can use sockets.
Have your application create a socket server and listen at some port in localhost. If it fails to listen, it would mean that someone else is listening to that port already - most likely, another instance of your app. You can even connect to that port and send messages to notify the primary instance that a second instance tried to be spawned.
The caveat is that if another app legitimately uses that port your app would never be able to run - but I find that very unlikely to happen.

There are many ways to go about this:
as you already figured, the file system can be used as communication channel between two jvms. But that only works for jvms running on the same server.
thus the already suggested socket solution enables you to (later) apply the same solution to a distributed environment. The downside is that you have to implement a protocol on a very low level.
in the past, people often turned to message bus solutions (think ActiveMQ for example)
in 2018, the other alternative would be to implement a simple restful API, using jaxrs and jersey for example.
As said: the effort you want to put into this depends on your requirements. How long will it be used? What are the odds that your solution will grow and has to scale to more than one server?!

try to use pidfile as lock and process single like kill 9 as communication tool

How to make Haxe communicate with Java(Processing 2) / Arduino?

I'm currently using Haxe with Flash target for a project.
What I want to do is to send a serial signal to my Arduino board, I couldn't find a way using only Haxe so I'm using Processing 2(http://processing.org/) which gets the job done.
The problem now is to make Haxe and Processing communicate, I've tried looking into memory mapped files, but Haxe and Java seem to work differently with those (Haxe needs an Int for address and Java uses a file).
And even creating a file to be read by processing and written by Haxe isn't working (Java gives a FileNotFound execption).
The main goal is to make Haxe send a signal to Arduino, I'm just using Processing 2 as a bridge, but any solution is welcome.
Do any one know a good way to solve this problem?
Thank you for the attention.

You can use sockets to communicate between software. Typically they are used over the internet, but it works great locally as well. For Processing, you would use the Network library, and it appears Haxe has a library as well.

apache httpclient syncronized execution

I want to make a test application in Java, what is testing what is happening when I do 2 post/put request at the same time to the same URI. I have written the putter and poster functions, I made a runnable class what generate test files, and run the functions in separated threads, at the same time. The problem is when I'm uploading a file what is bigger then 1Mb the network delay is ruin my measure. So I want to end the 2 execution very close.
I am using the Apache httpclient opensource library I try to searching in the source, but I'm not so good in reverse engineering. I think synchronizing the entry classes will not work. I tried to warp out the DefaultRequestDirector, but it is dead end to me too, because I'm not an expert in http protocols, and I don't understand what those functions do and why (but mostly why).
Can somebody tell me what classes and functions I need to modify if I want to leave open the uploading stream (or holding back the last pocket) until the other thread is reach the same point?

Advice on which language to use

I'm trying to create a web application which will get input from system.
What this application should do is to listen what happens when some shell scripts are executing and reporting the status trough web.
An example :
I'm copying thousands of records with shell script, and while this is still executing I'd like pass the current status of the script to the web interface. From shell script side I could echo something like "The files are being copied please wait for it to finnish".
Using which programming language from shell side would be the easiest way to pass this to the web interface? I intended to use JSP/JAVA for web.
Is this even the right way to think about this? Every suggestion is welcome
EDIT
Would something like http://nodejs.org/ be useful for this?

I'd use a named pipe (FIFO) instead. You simply write your output to the pipe and let the application read it. I'm not sure if there is any other way to get a more live system than this.
I'd recommend Perl as the back-end.
EDIT:
named pipes are a special type of files on UNIX. The abbreviation FIFO stands for "First In First Out". On LINUX Journal you can find an interesting read about named pipes.
Perl is a very powerful scripting language with many ready-to-use modules which you can find on http://cpan.org. You can find some answers here on SO about how/where to start learning Perl.

The Web part of your application can easily read a file or a database, so you just need to make sure that your shell scripts are outputting something for your Java code to update.
For example, if you run your shell script like this
./myscript.sh > mylog.log
Then in your Java code (note that you should not have logic in your JSP), you can read in the file to determine the status of the update, and output the result to your JSP.
It would be better to read the data in from a database, but that would involve you changing your shell script to output the data to a database.

You could put shell script output to some storage on server. And use AJAX on the web page to poll the server and get updates from that storage to the page.
If you will decide to use JSF for web page creation I can recommend "a4j:poll" component from ajax2jsf library. It is very simple and straightforward. Very easy way to poll server from the web page.

Writing to log file would be the simplest solution. And in programming, simple often means good.
If you really need very fast/realtime system, you should probably make these logs a database.
As for language, use what you like best. They all do support SQLite, including bash.

If I understand your question correctly you want to display a web page on a client machine that reports the status of a long running task on a server machine. If that is the case then you need to focus on something called AJAX IMO.
For what it is worth the simplest and easiest to understand implementation of AJAX that I know of is Apache Wicket.

web-browser based GUI

I am working on an application in Linux which will interfaces with hardware. One of the requirements is to create the GUI in Web-browser . the application will be c++ based. I m not familiar with web realted stuff so i want to know Is it possible to do such a thing (currently it's a console application take input from txt file/cmd line). gui will be simple using button and showing output messages on browser from the application. i want to know which technologies/languages are involved and how can it be done. some of the idea i read but havn't found anything concrete yet. if u have any idea about these or a better suggestion please share
run the app in background and communicate with browser ?
call library functions directly from browser ?
any other idea ?

I would start by setting up a regular HTTP server, like lighttp or Apache httpd.
You say you already have a command line program that does the actual work - As a first step, I would reuse that, and configure the web server to call your program using CGI - see forexample http://httpd.apache.org/docs/2.2/howto/cgi.html for apache
Finally, I'd pick some javascript framework like jQuery or YUI with Ajax capabilities to do requests to the server to call the CGI script from within a webpage. You could also create a form-based web application without ajax or any framework, but that would require you to stuff all kinds of logic in your program to generate HTML pages. By using Ajax, you can leave the command line application as is, and parse any responses it gives with javascript, and then use that to dynamically change the webpage in a way that would make sense to the user.
If this all works, then I would try to figure out how to package all these components. Perhaps you just want to create a simple archive with all the programs inside, or maybe you want to go as far as actually embedding the webserver in your program. Alternatively, you may want to do it the other way around and rewrite your program as an ISAPI module that you can plug into your webserver. Or if that's not integrated enough still you could write your own (partial) HTTP server. That's really up to you (I'd probably spend time and energy on searching for the leanest, meanest existing open source http serverr and use that instead)
At any rate, the prior steps won't be lost work. Most likely, developing the web page is going form a substantial part of the work, so I would probably create a quick and dirty working solution first using the age-old CGI trick, and then develop the webpage to my satisfaction. At that point you can already have an acceptable distributable solution by simply putting all programs in a single archive (of course you would have to tweak the webserver's configuration too, like changing the default port so it won't interfere with existing webservers.) Only after that I would spend time on creating a more integrated fancy solution.

I ended up using Wt though I'd update for future reference.

These are how I thought of doing this, in order of complexity for me:
Create a simple server-side-language (PHP/Python) website that can communicate with (ie launch and process the return of) your application
Modify your application to have a built-in webserver that just punched out HTML (command line parameters taken through the URL)
Modify the app to publish JSON and use javascript on a simple HTML page to pull it in.
You could write a Java applet (as you've tagged this thread) but I think you'd be wasting time. This can be quite simple if you're willing to spend 10 minutes looking up a few simple commands.

After 12 years, web browser-based GUI started to appear, WebUI is one of them.